Signal translating apparatus



Dec. 1942- D. H CUNNINGHAM SIGNAL TRANSLATING APPARATUS Filed March 1,1941 16.1.

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Patented Dec. 1, 1942 2,303,989 SIGNAL TRANSLATING APPARATUS David H.Cunningham, Haddonfield, N. J., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application March 1, 1941, Serial No.381,213

6 Claims.

This invention relates to signal translating apparatus and moreparticularly to a loudspeaker, the present invention being animprovement over that disclosed in the Carlisle Patent No. 2,007,750, ofJuly 9, 1935.

In the aforesaid patent, there is disclosed a loudspeaker of theelectrodynamic type in which a mechanical filter is incorporated forcontrolling the high frequency response. This filter consistsessentially of a compliant connection between the voice coil form andthe diaphragm and, in some instances, also includes a mass secured tosome part of the system, the arrangement being such as to provide apredetermined cutoff at the higher frequencies.

While an arrangement such as disclosed by Carlisle is very satisfactoryfor most practical purposes, there are some cases in which a sharpercut-off is desirable, and the primary object K of my invention is toprovide a loudspeaker which will accomplish this purpose.

More particularly, it is an object of my invention to provide, in anelectromechanical loudspeaker, an improved mechanical filter by means ofwhich more precise control of the high frequency response may beobtained than was heretofore possible.

Another object of my invention is to provide an improved acoustic filterwhich is extremely simple in construction, which is highly efiicient inuse, and which is exceedingly inexpensive in cost.

It is also an object of my invention to provide an improved mechanicalfilter for loudspeakers as aforesaid which may be applied readily tomanyexisting loudspeaker diaphragms.

According to my invention, the diaphragm, such as the frusto-conicaldiaphragms commonly in use, is provided with a circumferentialcorrugation or the like adjacent its smaller end or base to constitute acompliance therein, a driving coil being wound on a coil form secured tosaid smaller base in conventional manner. On a diameter greater than thecorrugation, I secure to the front surface of the cone a metallic ringwhich constitutes a mass, the corrugation in the diaphragm being locatedbetween the driving coil and the ring, and the three being preferably inconcentric relation. Thus far the system is, effectively, quite similarto that disclosed by Carlisle in the above-identified patent. Myimprovement consists in the addition of a cap to the aforesaid ring, thecap being made of a paper, Celluloid, or any other material whichpossesses relatively great inherent compliance in a direction axially ofthe cone. The mass of the cap and its compliance form a shunt resonantmechanical trap which provides an extremely sharp cut-off heretoforemade possible only by relatively expensive electrical filters, so far asI am aware, whereas the cost of my improved filter is at most only a fewcents. In addition to its acoustic function, the cap may also serve as adust cap in conventional manner.

The novel features that I consider character- Figure 2 is a wiringdiagram of an electric circuit equivalent to the acoustic system shownin Figure 1, and

Figure 3 is a response curve showing the response of my improvedloudspeaker as compared with the responses of (1) a conventionalloudspeaker and (2) a loudspeaker of the type disclosed in theabove-noted Carlisle patent.

Referring more particularly to the drawing, wherein similar referencecharacters designate corresponding parts throughout, there is shown, inFigurel, a frusto-conical diaphragm l to the smaller end of which issecured a cylindrical form 3 on which a driving coil 5 is wound, the

- coil 5 being disposed in the air gap 1 between a central pole piece 9and an outer pole piece I! of a suitable magnet system. is formedadjacent its smaller end with a circumferential corrugation or the likei3 which constitutes a compliant coupling between the coil 5 and themajor portion of the diaphragm, a centering suspension l5 of well knownform being secured to the diaphragm l preferably between the compliancel3 and the coil 5.

Secured to thefront surface of the diaphragm l on a diameter greaterthan that of the corrugation or compliance l3 and preferably inconcentric relation to the corrugation coil 5 is a ring ll of metalor-any other suitable material constituting a mass which loads thediaphragm l. The ring I! may be made of aluminum, steel, copper, hardrubber, Bakelite, or any other similar material having substantial mass,and it is secured to the diaphragm l by a suitable adhesive in aposition such that the com- The diaphragm I' I3 and the pliance I3 islocated between it and the coil 5. To the ring I! is secured a capmember [9 of a material which is fairly lightand has relatively greatinherent compliance, particularly in a direction axially of the cone.Such materials as paper, Celluloid, cellulose acetate, linen impregnatedor coated with a thin coating of a suitable resin, or the like, may befound suitable, but I preferably employ a soft paper having a thicknessof the order of 6 to mils. This paper may be of the same type, forexample, as is used for the diaphragm I, but not necessarily of the samethickness, being usually thinner thanthe diaphragm. The cap I 9 may bedome-shaped, as shown, or of any other shape, it being essential,however, that it should be compliant. If necessary, the cap [9 may evenbe formed with one or more corrugations to make it sufiicientlycompliant.

By reference to the equivalent electrical wiring diagram of Figure 2, aclearer understanding of the nature and operation of my improvedloudspeaker may be had. In this diagram, the voltage across the linecorresponds to the driving force f derived from the coil 5, and theseries connected inductors M1 and M2 correspond, respectively, to themechanical impedances due to the mass of the coil 5 and the mass of thering I]. The capacitor C1 shunted across the line between the inductorsM1 and M2 represents the compliance of the corrugation t3, the linebeing terminated by the resistor Z which represents the mechanicalimpedance of the cone I. Thus far, the system is quite similar to thatrepresented in Figure 6 of the above-noted Carlisle The addition of thecompliant cap l9 introduces into the system, in series with theinductors M1 and M2 and the resistor Z, the shunt resonant circuitconsisting of an inductor M3 and a capacitor C2, the inductor M3corresponding to the mechanical impedance due to the effective mass ofthe cap l9 and the capacitor C2 representing the effective distributedcompliance thereof. By proper selection of the values of M3 and C2across the shunt resonant circuit can be made to be maximum at thatfrequency and thereby sharp cut-off obtained.

The response curve of Figure 3 shows, by comparison, the advantages ofmy improved loudspeaker over those of the prior art. In this figure, thedotted curve A shows the response of a conventional loudspeaker conehaving neither the ring H nor the cap 19 thereon, The dot and dash linecurve B shows the response of a loudspeaker such as shown in Figure 7 ofthe Carlisle patent, that is, one having a mass similar to the ring llsecured thereto. It will be noted from curve B that the cut-off of sucha loudspeaker is sharper and the attenuation greater than in aloudspeaker having a diaphragm which is not loaded by a mass. Finally,the solid curve C shows the effect of adding the compliant cap I9 to thesystem, and it will be seen from this curve that not only is the highfrequency response below cut-01f extended somewhat, but the cut-off isalso considerably sharper and the attenuation much loudspeakers to whichthe curves A and B apply.

From the foregoing description, it will be apparent that I have providedan improved loudspeaker having the advantages heretofore noted. It willalso undoubtedly be apparent to those skilled in the art that, althoughI have shown and described my invention as applied to a loud patent.

for a particular frequency, the impedance greater than with either ofthe skilled in the art.

'ber secured to said annular speaker of the electrodynamic type and oneemploying a cone diaphragm, my improved filter is equally wellapplicable to loudspeakers employing other types of driving motors, suchas magnetic, condenser, piezoelectric, and the like, as well as todiaphragms of any other form. Also, instead of forming one or morecorrugations I3 in the diaphragm to give it compliance between the coil5 and the ringll, it is obvious that the diaphragm may be made of amaterial which will have suficient inherent distributed compliance inthis region for the required purpose. Other similar changes will, nodoubt, readily suggest themselves to those I, therefore, desire that myinvention shall not be limited except insofar as is made necessary bythe prior art and by the spirit of the appended claims.

I claim as my invention:

1. In a loudspeaker, the combination of a diaphragm, driving membertherefor, a mass secured to said diaphragm, said diaphragm havingcompliance therein between said driving member and said mass, and a capmember secured to said mass, said cap member being made of a materialhaving relatively great inherent compliance.

2. In a loudspeaker, the combination of a diaphragm, a driving membertherefor, an annular member constituting a mass secured to saiddiaphragm concentrically with and in spaced re-- lation to said drivingmember, said diaphragm having compliance therein between said drivingmember and said annular member, and a cap secured to said annularmember, said cap being made of a material having relatively greatinherent compliance.

3. In a loudspeaker of the electrodynamic type, the combinaticn of adiaphragm, a driving coil therefor, an annular member constituting amass secured to said diaphragm concentrically with and in spacedrelation to said coil, said diaphragm having compliance therein betweensaid coil and said annular member, and a cap memmember, said cap memberbeing made of a light-weight material having relatively great inherentcompliance.

4. In a loudspeaker of the electrodynamic type, the combination of adiaphragm, a driving coil therefor, a metallic ring constituting a masssecured to said diaphragm concentrically with and in spaced relation tosaid coil, said diaphragm having compliance therein between said coiland said ring, and a paper cap secured to said ring, said cap havingrelatively great inherent compliance.

5. In a loudspeaker of the electrodynamic type, the combination of afrusto-conical diaphragm, a coil form secured thereto at its smallerend, a driving coil for said diaphragm carried by said form, saiddiaphragm having a circumferential like therein in proximity tovcorrugation or the said smaller end and constituting a compliance, aring of greater diameter than said compliance and constituting a masssecured to said diaphragm concentrically with said compliance andsaidcoil, and a cap member secured to said ring, said cap member being madeof a material having relatively great inherent compliance.

6. The invention set forth in claim 5 charac terized in that the mass ofsaid cap member and the compliance thereof provide a shunt resonantmechanical trap which imparts a sharp;

cut-off to said. loudspeaker.

DAVID H. CUNNINGHAM. I

